The present invention relates generally to the field of data tape transport devices and, in particular, to controlling the braking of a tape transport motor during a power failure in such a fashion as to prevent damage to the tape.
Designers of tape transport systems face the problem of how the transport should respond when an emergency stop of tape motion is required. Especially in high speed tape transports, it is essential to decelerate and stop tape in a safe manner when a power loss occurs or when an error is detected. In either type of situation, the servo control system governing tape motion may be disabled due to the power loss, leaving the tape to coast to a stop. However, uncontrolled tape motion may cause loose tape to be created in the tape path.
Numerous methods of stopping a tape after a power loss have been proposed. In one such method, a friction brake is engaged to stop the tape transport motors. Another method uses back-EMF from the motors to provide power to a control circuit which, in turn, provides braking control for the two transport motors. Still other methods rely on a back-up power supply, such as a battery, to provide power for the control circuit in the event of a power failure.
When tape is moving at high speed, such as during rewind, locate to a file, or read and write operations, a powerloss may cause high tape tension to develop, resulting in tape damage. As previously noted, loose tape in the tape path may also occur during a power loss. Excess tape tension or loose tape both increase the risk of tape damage during the emergency stop or during a subsequent recovery of the tape transport to unload the tape carrier. Physical damage to the tape may result in the undesirable loss of customer data recorded on the tape.
Additionally, certain events, not related to power loss, may necessitate the shut down of a tape transport. Such problems may include a controller firmware glitch, detection of a problem with the tape tension or an unexpected reboot of the microprocessor or controller.
Thus, there remains a need for a system and method to control the deceleration of tape drive motors in the event of a power failure in any of the several voltages which may supply power to any circuit used to control the tape drive motors. There also remains a need for a system and method to control the deceleration of tape drive motors upon detection of non-power related emergency events. Importantly, such a system and method should maintain the tension of the tape media within manufacturers specification during the deceleration of tape drive motors in order to prevent damage to the tape.
The present invention provides an apparatus and method for the controlled dynamic braking of a DC motor in a tape transport device. The apparatus includes a power-fail detection circuit coupled to detect a low power event in any of at least one primary power supply, pulse width modulation (PWM) state device and an energy storage device to power the PWM state device during a low power event. The PWM state device includes a plurality of state transition paths. Each path has an initial state, representative of a tape profile, including a predetermined range of tape velocities, relative amounts of tape on each reel, an initial tape tension, and at least one subsequent state. A first input to the PWM state device receives a trigger signal from the power-fail detection circuit and a second input receives a profile signal indicative of the present tape profile. When a low power event is detected, one of the state transition paths is selected and an associated PWM signal is transmitted to the motor. The PWM signals follow the selected transition path to modulate the current generated by the motion of the motor and thereby create a decelerating force while substantially maintaining the initial tape tension.
In one embodiment, the PWM state device is coupled to, and receives control signals from, a control register. The control register may receive signals, such as the profile signal, from a microprocessor. Moreover, when the motor control circuit includes a power switch (such as an FET) for each motor winding, the PWM signal modulates the power switches to sink motor-generated current to create the decelerating force.
In another embodiment, controlled dynamic braking may be initiated by the microprocessor in the absence of a low power event.